Somehow it looks like ctrl is pressed all the time.
These symptoms don't come togheter.
The 'mouse can' t leave' - problem occurred only twice.
These symptoms usually occur while many tabs are open.

Is anyone else experiencing similar behaviour?

btw:
If you are using a sandboxed firefox as your default browser through
'System' -> 'Preferences' -> 'Preferred Applications' -> Internet ->
Command
consider double quoting %s in the command -> '"%s"'
or you might just get a black Xephyr window without firefox if your URL
contains special chars. I guess this happens because these parameters
get passed around (sandbox -> sandboxX.sh).
Don't know if this should be considered a bug.